<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }

        body {
            height: 100vh;
            background-color: #FFCCCC;
        }

        .main {
            position: relative;
            background-color: antiquewhite;
            width: 80%;
            top: 100px;
            left: 50%;
            transform: translate(-50%, 0);
            /* margin: 100px auto 0; */
        }

        .flex {
            display: flex;
        }

        .child {
            height: 100px;
            flex: 1;
            background-color: #CCFFFF;
            margin: 5px;
        }
    </style>
</head>

<body>
    <div class="main">
        <div class="flex">
            <div class="child"></div>
        </div>
        <div class="flex">
            <div class="child"></div>
            <div class="child"></div>
        </div>
        <div class="flex">
            <div class="child"></div>
            <div class="child"></div>
            <div class="child"></div>
        </div>
        <div class="flex">
            <div class="child"></div>
            <div class="child"></div>
            <div class="child"></div>
            <div class="child"></div>
        </div>
    </div>

</body>

</html>